|JOHNSON, J - University Of Kentucky|
|SCHRICK, F - University Of Tennessee|
Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/19/2013
Publication Date: 12/1/2013
Citation: Aiken, G.E., Klotz, J.L., Johnson, J.M., Strickland, J.R., Schrick, F.N. 2013. Postgraze assessment of toxicosis symptoms for steers grazed on toxic and novel endophyte-infected tall fescue pasture. Journal of Animal Science. 91:5878-5884.
Interpretive Summary: An endophytic fungus found in tall fescue plants produces ergot alkaloids that cause fescue toxicosis in cattle. Rough hair coats during the warm season, elevated core body temperatures, labored breathing, and decreased prolactin concentrations are symptoms of fescue toxicosis. Elevated core body temperatures are a result of the binding of ergot alkaloid toxins to biogenic amine receptors to restrict blood flow through the peripheral vasculature and thereby impair the animal’s ability to dissipate body heat. Consequently, cattle consuming ergot alkaloids suffer from severe heat stress in the presence of high ambient temperature and humidity. Endophytes have been collected, screened, and developed that do not produce ergot alkaloids, but do produce other alkaloids that impart tolerances to environmental stresses. A pen experiment was conducted for two years following the grazing of toxic endophyte infected ‘Kentucky 31’, novel endophyte [‘Jesup’ infected with AR 542 in yr 1 and KYFA9301 infected with AR584 in yr 2), and endophyte-free KYFA9301 tall fescues. Results of the experiment indicated that declines in rectal temperatures in cattle following destocking of toxic endophyte-infected tall fescue pastures and their placement on nontoxic diets are associated with prevailing moderate to low ambient temperatures, and not from relaxation of vascular constriction. Prolactin concentrations in steers grazed on toxic endophyte-infected fescue indicated a recovery in 15 d as compared to those of steers grazed on endophyte-free and the 2 non-toxic endophyte-infected fescues. Use of rectal temperatures and serum prolactin concentrations as measures of fescue toxicosis may not be accurate as indicators of complete recovery. Information generated by this research will have an impact on feedlot managers and cattlemen that are retaining ownership of calves from tall fescue pasture to the feedyard.
Technical Abstract: A 2-yr pen experiment was conducted using 18 crossbred Angus steers each year to evaluate changes in body temperature, vasoconstriction, and prolactin concentrations in steers previously grazed on toxic endophyte (Neotyphodium coenophialum)-infected Kentucky 31 tall fescue (Lolium arundinaceum) as compared to those previously grazed on novel endophyte [‘Jesup’ infected with AR 542 (MaxQ) in yr 1 and KYFA9301 infected with AR584 (NE9301) in yr 2], and endophyte-free (EF9301) tall fescues. Groups of 6 steers for each grazing treatment were blocked by BW for assignment to 2 pens as a randomized complete block design with 2 replications. Two environments were implemented by initiating the experiment on 18 August in yr 1 and on 8 September in yr 2 for durations of 30 and 21 d, respectively. Rectal temperatures were recorded and jugular blood was collected for assaying serum prolactin, and cross-sections of the caudal artery were ultrasonically imaged at selected time points. Initial rectal temperatures and prolactin concentrations were similar (P > 0.10) between EF9301 and novel endophyte fescue steers in both yr. Rectal temperatures of KY31 steers declined (P = 0.011) curvilinearly in yr 1 and were similar (P > 0.10) to MaxQ steers from days 20 to 30, and to EF9301 steers on the final day of monitoring (day 30). Rectal temperatures of KY31 steers in yr 2 declined linearly (P < 0.001) and were similar (P > 0.10) to NE9301 and EF9301 steers at days 15 and 20. Prolactin concentrations in yr 1 in KY31 steers showed curvilinear increases (P < 0.05) over time and were similar (P > 0.10) to MaxQ steers from days 10 to 30, and to EF9301 steers by day 15. In yr 2, curvilinear increases of prolactin concentrations in KY31 steers were similar (P > 0.10) or greater (P < 0.05) than those in NE9301 and EF9301 from days 5 to 20. Luminal areas of the caudal artery were similar (P > 0.10) between steers that grazed EF9301 and the 2 novel endophyte pastures, but luminal areas in KY31 steers were less (P < 0.05) than those in EF9301 and novel endophyte steers across all dates in both years. Results indicated there was no alkaloid-induced vasoconstriction in steers that grazed either of the 2 novel endophyte-infected fescues. Prolactin in steers after they are removed from toxic fescue may increase and stabilize in less than 2 wk, but alkaloid-induced vasoconstriction is not alleviated within 30 d.